Sky Lights

Most of the cosmos cannot be seen, but its influence is visible everywhere you look

The recent news that most of the universe is invisible poses a philosophical quandary. One of the great joys in gazing at the night sky comes from feeling a connection with the cosmos. But the latest theories hold that 96 percent of everything up there is in the form of exotic dark matter and dark energy that emit no light and cast no shadows. How do you connect with a universe you cannot see?

WHAT’S UP IN THE

OCTOBER SKY

Evening skies this month provide fine views of stars and galaxies. For planets, set your alarm: All the bright ones lie in the morning sky.

OCTOBER 1

Jupiter emerges from the sun’s glare, climbing steadily in the predawn east.

Dark energy in particular challenges the imagination. This hypothetical essence appears to be causing the universe to blow apart at an accelerating tempo, an explanation for why the universe appears to be expanding more quickly today than it was several billion years ago. Dark energy influences the cosmos only on the very grandest scales, however. On a more local level, it is too diluted to make itself felt. Even at the extreme range of the unaided human eye—about 2.9 million light-years to the galaxy M33, now low in the east after the end of twilight—dark energy is less than invisible. It has no evident impact of any kind.

Dark matter is a different story. Swiss astronomer Fritz Zwicky deduced its presence more than 70 years ago when he realized that a powerful gravitational pull from some unknown, unseen substance seemed to bind together clusters of galaxies. In this case, the effects show up clearly in our own cosmic neighborhood. Without dark matter, our local group of galaxies would disperse, and the famed Andromeda galaxy would not hover overhead on autumn evenings.

A small oval smudge of light easily seen from any dark site in the Northern Hemisphere, Andromeda is accompanied by small galaxies, two of which can be seen with a powerful pair of binoculars. The Milky Way is likewise escorted by at least a half dozen small galaxies trapped by the gravitational epoxy of dark matter, which is scattered all through and around our galaxy’s luminous stars.

These dwarf galactic companions testify to another effect of dark matter, says astronomer Andrey Kravtsov of the University of Chicago. “It’s difficult to understand how such small galaxies should exist at all,” he says. In June 2003 he and his collaborators analyzed supercomputer simulations that provide a partial explanation.

Many of the minigalaxies probably started out much larger, containing enough dark and luminous matter to evolve into cohesive systems of stars. Over time, these galaxies grew smaller and dimmer as they were cannibalized by their more massive neighbors. Kravtsov’s simulations support the idea that dark matter is made of weakly interacting massive particles, or WIMPs—hypothetical particles so unreactive that they can pass right through ordinary matter. In fact, such particles could be passing through your body right now. WIMPs would be slow moving and unaffected by radiation (“cold dark matter” in scientific terminology), so they could easily gather into giant clumps that would help draw together the bright parts of galaxies.

Dark matter also makes itself felt within the Milky Way. The gravitational tug from invisible matter dramatically influences the way the galaxy spins. Without it, stars closer to the galactic center—in the direction of Sagittarius, glowing prominently in the south during the moonless nights of early October—would orbit the nucleus more quickly than would the sun. Conversely, stars farther from the core—such as those in Orion, rising by midnight this month—would get left behind. Because of the influence of dark matter, the Milky Way’s components all seem to move at about the same speed. As a result, the stars that surround us today will mostly continue to keep us company for millions of years to come.

If cosmologists are correct, every single point of light in the night sky bears the imprint of dark matter. It’s not just that the constellations tend to stick together or that groupings of galaxies remain intact in the face of the universe’s overall expansion. Without a gravitational boost from dark matter, the hot gases that emerged from the Big Bang would have evolved into a misty universe devoid of galaxies, stars, planets, or any notable structure at all. Want to feel connected to the

Astronomer Fritz Zwicky discovered dark matter and predicted that foreground galaxies could magnify the images of more distant objects. It took half a century to confirm these ideas, yet Zwicky’s name is little known today—largely because his abrasive personality won him few friends.

dark universe? Its influence lies not just in the stars above but also in the earth beneath your feet.